Video artifacts

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Artifacts are visible distortions that were not in the original picture. Learning to name them makes it much easier to judge an encode and to pick the right fix. A useful first split: spatial artifacts are visible in a paused frame; temporal artifacts mostly show while the video plays. Check both, at normal speed and frame by frame.

Not every artifact comes from the encoder. Moiré, jagged edges, halos and some banding are often already in the source, introduced by the camera, scaling or earlier processing. Compare with the source before blaming your settings.

Blocking

Block-based codecs split the picture into blocks and code each one with a transform such as the DCT. When too few bits are available, each block is reduced to a coarse approximation, and the edges between neighboring blocks no longer line up, leaving a visible grid. Deblocking filters in H.264 and later codecs hide most of this, so in modern encodes blocking mainly shows at very low bitrates, in fast motion, and in dark or flat areas.

Banding

Banding (contouring) turns smooth gradients such as skies, walls and dark fades into visible steps. Coarse quantization flattens the subtle variations and fine noise that kept the gradient smooth, and 8-bit precision leaves few levels to work with in dark scenes. Encoding at 10 bits, giving flat areas more bits through adaptive quantization, and keeping a little grain or dither all help. If the source already has banding, a deband filter can fix it before encoding.

Ringing

Ringing appears as ripples or faint halos running parallel to sharp edges, like the colored fringes around the star above. A sharp edge needs many high-frequency coefficients; when quantization removes some of them, the edge is rebuilt with oscillations around it. Line art and text show it most. Similar halos also come from sharpening and some resizing filters; see dehalo.

Mosquito noise

Mosquito noise is the temporal relative of ringing: busy, shimmering specks around edges and fine detail that change from frame to frame, like insects buzzing around the edge. The quantization error near the edge is different in each frame, so it flickers during playback even when each frame looks acceptable on its own.

Color bleed

Color bleeds or smears across edges into neighboring areas. Most video stores color at lower resolution than brightness (chroma subsampling), so color edges are softer and less precise than luma edges, and low bitrates smear them further. Saturated reds and blues against dark areas show it most.

Blur and loss of detail

The most common artifact in modern encodes is not blocks but smoothness: grain, skin texture, foliage and fine fabric become flat or waxy. Squared-error decisions favor blur because it is cheap and scores well on simple metrics. Lower CRF values, psychovisual options, and film-grain synthesis for grainy sources counter it.

Pulsing and flicker

Quality that visibly “breathes” in a regular rhythm usually lines up with keyframes or the encoder’s frame hierarchy: some frames get more bits than their neighbors, and texture sharpens and softens as they pass. It is easiest to see in static shots with grain or fine texture. More bits help, as do encoder options that even out quality between frame types, such as SVT-AV1’s --qp-scale-compress-strength.

Moiré

Moiré is a large, swirling interference pattern over fine regular detail such as brickwork, fabric or screens. It appears when that detail is sampled at too low a resolution, by the camera sensor or by downscaling with a poor filter. Compression can exaggerate it, but the usual cause is sampling or scaling.

Staircase effect (aliasing)

Diagonal and curved edges that should be smooth look jagged, like stair steps. This is aliasing, usually from scaling, poor deinterlacing or low-resolution rendering rather than from the encoder. Anti-aliasing filters reduce it.